1. Field of the Invention
The present invention relates to a high-frequency (HF) circuit device and a manufacturing method thereof. Particularly it relates to an HF circuit device having a good HF characteristic and having a substrate structure in which transmission loss of an HF signal is small and a manufacturing method thereof.
2. Description of the Background Art
In recent years, the use of an HF circuit device using GHz band for a movable telephone has spread. With the spread of the use of the HF circuit device, there is a greater need for lowering the cost of the HF circuit device. In order to satisfy the need, it is desired that the cost of an integrated circuit device for processing signals used in the HF circuit be lowered.
In a convectional signal processing circuit accommodating a signal of GHz band, an expensive semiconductor device using, for example, GaAs has been used. However, for higher integration and lower cost of the circuit, shifting to a silicon device is desired. As a recent silicon device is further miniaturized and has its HF characteristic improved, a similar performance to that of the conventional GaAs device is being implemented in a circuit arrangement using a bipolar device or a CMOS circuit.
However, while the performance of the silicon device has been improved as described above, signal loss through a silicon substrate is a problem in terms of integrated circuit structure, since the silicon substrate is conductive to HF signals. For the GaAs device described above, since an active element is formed on a GaAs substrate as an insulator, dielectric loss in the substrate is small. Accordingly, an integrated circuit having a superior HF characteristic will be obtained.
Now, referring to FIG. 32, an example of an HF circuit device using a conventional, general silicon substrate will be described. FIG. 32 is a schematic cross sectional view of an example of a conventional HF circuit device using a silicon substrate.
Referring to FIG. 32, an oxide film 3 is formed on a silicon substrate 11c having a specific resistance of approximately several 10 .OMEGA.cm. A metal interconnection layer 4 is formed on oxide film 3. An oxide film 5 is formed such that it covers metal interconnection layer 4. On a back surface of silicon substrate 11c, a back-surface metal interconnection layer 6 is formed, since solder is used at the time of assembling.
By applying an HF signal to the interconnection of the HF circuit device using the conventional silicon substrate as described above, a line of electric force will extend into silicon substrate 11c. Accordingly, as shown in FIG. 32, loss of resistance component Ri and loss Rm between metal interconnection layers 4 in silicon substrate 11c are equivalently added, and signal loss and resistive coupling components between the interconnections will appear. Such a loss results from the fact that silicon substrate 11c is a dielectric having loss.
Generally, dielectric loss tangent tan .delta. is used to express the amount of loss described above. The dielectric loss tangent tan.delta. is defined by complex dielectric constant (real number part/imaginary part). The real number part for the numerator corresponds to the resistance loss. Therefore, if as the value of tan .delta. is larger, the resistance loss increases. For silicon substrate 11c generally known as having a specific resistance of several ten .OMEGA., the value of tan .delta. is approximately two to three. On the other hand, for a GaAs substrate, tan .delta. is as small as approximately 0.001. For this reason, there has been a problem that transmission loss of HF signals becomes greater when silicon substrate 11c, which has been conventionally used in general, is used simply in place of the GaAs substrate.